Our group has been investigating these HVCCs for 20 yr extensively, particularly being interested in those without any apparent driving source. There are few reports in the Galactic disk, while \(\sim \!80\)  HVCCs have been identified in the CMZ so far. We found an extraordinary broad-velocity-width (\(\Delta V\!\! \simeq \!\!100 \)  km s\(^{-1}\) ) feature in the W44 giant molecular cloud (Bullet; [1]}). This is only one example of an HVCC-like feature in the Galactic disk. The huge kinematic energy of the Bullet, which cannot be accounted for by the W44 supernova (SN), and its “Y"- shape feature in the position-velocity plane can be explained by the high-velocity plunge of an isolated black hole into the dense molecular gas layer behind an SN blast wave ([2]}; [3]}).
